1. Field of the Invention
The present invention relates to a radar technique, and more particularly, to an effective technique being applied to detect a target positioned in a wide range from a long to short range.
2. Description of the Related Art
Attention is paid to a millimeter wave radar technique intended to observe, for example, an obstruction on a road, and a traffic condition in operation management the car with the radar.
A conventional FM-CW (Frequency Modulation-Continuous Wave) radar system can detect a target in a distance range of several meters to a hundred and several tens of meters, but has a difficulty in the detection of a target in an extremely short range such as approximately 10 centimeters with the same degree of accuracy. In the meantime, an ultra-short pulse radar system can detect a target in a distance range of approximately 10 centimeters to a ten and several meters with high accuracy, but has a difficulty in the detection of a target in a distance range of 100 meters or more.
As described above, an FM-CW radar and an ultra-short pulse radar are totally different radar systems. Accordingly, two different radars are conventionally required to comprise detection performance in long and short ranges of a distance.
A long- and short-range radar 300 into which two radar apparatuses are combined is exemplified in FIG. 1 as a reference technique of the present invention. FIG. 1 shows an example where a long-range radar 302 is an FM-CW radar according to a conventional technique, a short-range radar 303 is a pulse radar using an UWB (Ultra Wide Band) technique, detection signals from both of the radars are combined by a integrated unit 301, and detection results covering from long to short ranges are externally output.
As the long-range detection radar, an FM-CW radar is most common. Non-Patent Document 1 discloses a radar of an RF frequency of a 76-GHz band, and a detection distance range of 4 to 120 meters.
Short-range radars are under development mainly in Europe and the United States. For example, like Non-Patent Document 2, a UWB system of an RF frequency of a 24-GHz band and a distance of approximately 0.1 to 20 meters is targeted.
The detection system that covers a short to long range and is configured by simply using such two radars according to these existing techniques is the above described FIG. 1. Such a system has a disadvantage that its configuration becomes complex, and its cost becomes expensive in comparison with a case where a detection system is implemented with one apparatus.
In the meantime, Patent Document 1 discloses a radar transmitter/receiver for detecting a long-range target with an FM pulse radar system, and for detecting a short-range target with an FM-CW radar system.
However, as recited in the above described Non-Patent Documents 1 and 2, the FM-CW radar system is effective for a long range of 3 to 100 meters, whereas the pulse radar system is effective for a short range of 0.1 to 20 meters. Therefore, the radar transmitter/receiver disclosed by Patent Document 1 is inconsistent with the common knowledge of techniques in this field. Namely, for the FM-CW radar, if a detection distance is very short such as 1 meter or less, a beat frequency obtained from transmission and reception signal becomes very low, and it becomes difficult to accurately count the frequency. Additionally, it is difficult to discriminate the two targets that approach in a range of several tens of centimeters. For example, if 20 centimeters is required as a discrimination of a distance, a frequency modulation deviation is approximately 750 MHz. However, it is very difficult to manufacture the linear FM modulator which has low modulation distortion with the frequency deviation of 750 MHz, and an FM oscillator for implementing this FM modulation becomes very expensive. Furthermore, it is difficult to implement an FM modulator that is claimed by Patent Document 1, and makes high-speed FM switching with a short pulse, and its radar system and configuration are different from those of a system which can implement a pulse radar mode with amplitude modulation according to the present invention.
[Non-Patent Document 1] “Fujimoto and Ida, “Millimeter Wave Car-Mounted Radar System, NEC Technical Journal Vol. 54, No. 7/2001”, Jul. 25, 2001
[Non-Patent Document 2] Andre zanderVolkswagenAG et al., “A Multifunctional Automotive Short Range Radar System”, [online] [search on Jul. 1, 2004], <URL: http://www.smart-microwave-sensors.de/GRS—2000_Multifunctional_Short_Range_Radar_Systems.pdf>
[Patent Document 1] Japanese Patent Publication Laid-open No. 11-258340